1. Field of the Invention
The present invention relates to novel polyamide-based compositions having improved properties of impact strength, ductility and crystallinity, together with markedly enhanced heat stability.
2. Description of the Prior Art
Unmodified polyamides which possess high break strengths are considered to be tough polymers. In contrast, same are typically rather lacking or deficient as regards cracking and resistance to the propagation of cracks, which is the result of a certain sensitivity to chipping or notching and, possibly, of breakage due to embrittlement. This tendency to break as a result of being brittle rather than ductile substantially limits the uses of such polyamides and their reliability.
Improvement in the impact strength of thermoplastic polyamides has been studied in detail and numerous solutions to the problem have been proposed. Thus, British Pat. No. 998,439 features a composition comprising a mixture of 50 to 99% of a linear polyamide and 1 to 50% of an olefinic copolymer containing from 0.1 to 10 mol percent of acid functions, dispersed in the polyamide in the form of particles having a diameter of less than 5 microns.
U.S. Pat. Nos. 3,388,186 and 3,465,059 feature mixtures of polycaproamide and olefin copolymers containing from 1 to 20 mol % of (meth)acrylic acids or derivatives thereof, onto which an aminoacid has been grafted.
U.S. Pat. No. 3,668,274 describes mixtures of polyamides and 2.5 to 30% of a multiphase polymer comprising carboxyl functions, consisting of a first elastomeric phase (50 to 99.9% of an alkyl acrylate and butadiene) onto which a more rigid copolymer, containing from 1 to 50% by weight of an unsaturated carboxylic acid, has been grafted.
U.S. Pat. No. 3,845,163 describes mixtures of 60 to 80% by weight of a polyamide and 15 to 40% by weight of an .alpha.-olefin copolymer containing from 1 to 8 mol % of an .alpha.,.beta.-ethylenic carboxylic acid in which at least 10% of the acid functions have been neutralized by metal ions. French Pat. No. 2,311,814 features a very large number of multiple-phase polyamide compositions having improved toughness, which consist of mixtures produced in the molten state from 60 to 99% by weight of a polyamide and 1 to 40% by weight of at least one other phase containing particles of at least one polymer having a modulus under traction of less than 1/10 of that of the polyamide, which "adheres" to the polyamide and the particle size of which is between 0.01 and 3 microns. The polymers which can be used as the disperse phase are selected from among the non-crosslinked, linear or branched chain thermoplastic or elastomeric polymers belonging to the following chemical families:
(i) Copolymers of unsaturated monomers and molecules which generate adherent sites, such as: carbon monoxide, carboxylic acids having .alpha.,.beta.-ethylenic unsaturation, or derivatives thereof, unsaturated epoxides and residues of an aromatic sulfonylazide substituted by carboxylic acids; PA1 (ii) Polyurethanes derived from polyester-glycols or polyether-glycols; and PA1 (iii) Polyether network polymers obtained by reacting epoxide monomers. PA1 (a) Acid compounds consisting of from 1 to 100% of dimeric acid and from 0 to 99% of another diacid, or of a mixture of diacids or other difunctional reactants which generate acid groups or are derived from acid groups and can contain other groups, which compounds are chemically stable under the polymerization conditions and preferably have a number of carbon atoms which is less than 13 in the case of the diacids and less than 25 in the case of the difunctional derivatives; PA1 (b) A short-chain linear, branched or cyclic diol having a number of carbon atoms which is preferably between 2 and 8, and/or a polydiol, such as polyethylene glycols or polytetrahydrofurans having a molecular weight which is preferably between 500 and 3,000; and PA1 (c) A preferably short-chain diamine having a number of carbon atoms which is less than or equal to 12, or a mixture of such diamines, it being possible for such diamines to contain an inert hetero-atom, such as, for example, oxygen or sulfur, in their chain. PA1 .eta.inh=4.6 (log t.sub.1 -log t.sub.o) PA1 t.sub.1 =flow time of the solution PA1 t.sub.o =flow time of the solvent PA1 .eta.inh: 1.173 dl/g PA1 COOH end groups: 66.07 PA1 NH.sub.2 end groups: 45.83 PA1 Notch impact strength: 0.56.+-.0.05 J/cm.sup.2 PA1 Flexural modulus: 2,400.+-.200 MPa/mm.sup.2 PA1 Tg by thermomechanical analysis: 55.degree. PA1 Mp: 260.degree. C. CpC=210.degree. C. .DELTA.=50.degree. C. tg .alpha.=17 PA1 Proportion of amide groups per 100 g: 0.885
The majority of the polymeric additives mentioned or noted in the prior art have a relatively low thermal or chemical stability, and this limits their use in the fields of conventional polyamides, in which the conditions of preparation or processing are very harsh. Thus, the polyurethanes and polyether network polymers described in French Pat. No. 2,311,814, supra, can reasonably be used only in conjunction with matrices having melting points below 200.degree.-220.degree. C.
The very large number of copolymers of olefins and acrylic derivatives described in the prior art also undergo very substantial decomposition when they are mixed, in the molten state, with polyamides having a high melting point, such as, for example, polyhexamethylene adipamide.
The use of polyesteramides which are at least partially obtained from dimeric acid, as a reinforcing phase for thermoplastic compositions comprising a polyamide matrix, has not been described heretofore. German Offenlegungsschrift No. 2,635,226 discloses solventless printing inks based on polyamide and/or polyesteramide. However, these mixtures have low melting points and a low viscosity in the molten state. Mixtures of this type thus have properties diametrically opposed to those featured herein and involve the use of means different from those hereby proposed.
French published Application No. 78/08,391 discloses compositions which consist of a polyamide prepared from .omega.-aminocarboxylic acids or lactams, and of a random polyetheresteramide prepared from .omega.-aminocarboxylic acids or lactams, an .alpha.,.omega.-dihydroxypolytetrahydrofuran having a molecular weight of 160 to 3,000, and a dicarboxylic acid. However, compositions of this type do not possess both high rigidity and high resilience. Furthermore, the structure of the polyetheresteramide described is fundamentally different from that of the polyesteramides consistent herewith.